Pulled directly from the encounter table.
Foothills north combat encounters, assuming all unlocks:
-Harpies
-Marked Mercs (doubled if sugo's quest is active to get iron)
-Vaush (removable)
-Manticore
-Jen (beat MMM)
-Arona (level 3 or above)
-Grettel (exclusive with Arona)
-Byvernia repeat (only appears if RoQ is done)
-Painted demon (after WC)
West:
-Harpies
-Wyvern
-Vaush(removable)
-Berwyn
-Arona
-Grettel (exclusive with Arona)
-Byvernia repeat (only appears if RoQ is done)
-Painted demon
Snipping away all the potential factors:
North:
*Did not unlock Jen, remove Vaush, before WC, level 3, Sugoquest not active
-Harpies
-MMM (non multiplied chance)
-Manticore
-Arona
West:
-Cleared Berwyn, remove Vaush, Byvernia cannot spawn before Right of Conquest done, before WC, level 3
-Harpies
-Wyvern
-Arona
Your best bet is to search foothills west because the MMM and Jen are major sources of encounter dilution, especially the MMM who can have a 2x spawn rate depending on quest status. Having Sugoquest active and Jen unlocked means Arona spawns are 1/2 that of foothills west.